The present invention relates to a vertical autoclave which can be used to carry out any operations involving a medium comprising a liquid phase and/or a solid phase and especially for the bulk manufacture of vinyl chloride-based polymers and copolymers. It also relates to a process for the bulk manufacture of vinyl chloride-based polymers and copolymers, implemented with the use of the said autoclave.
The bulk polymerization of a monomer composition based on vinyl chloride starts with a liquid monomer composition. Because of the insolubility of the polymer or of the copolymer in the monomer composition, the reaction product separates out in a solid state during the polymerization, in the form of particles which are kept suspended in the liquid monomer composition by the application of agitation; as by stirring, to the reaction medium. When the polymer or copolymer content in the reaction medium reaches a value of the order of 15% by weight, its consistency becomes similar to that of stirred curdled milk. This consistency increases until the monomer composition no longer forms a continuous liquid phase, and this occurs as soon as the polymer or copolymer content in the reaction medium reaches a value of the order of 25% by weight, beyond which value the reaction medium is in a pulverulent state. The reaction medium is maintained in this state by virtue of the continuation of the agitation, as by stirring, which prevents the reaction product from setting solid, until the polymer or the copolymer is obtained in a pulverulent state at the end of polymerization, after the unreacted monomer composition has been degassed.
In the present application, the expression "monomer composition based on vinyl chloride" or more briefly, "monomer composition" refers to vinyl chloride by itself or mixed with at least one other monomer which can be copolymerized with vinyl chloride. The said monomer composition based on vinyl chloride contains at least 70% by weight of vinyl chloride. As monomers which can be copolymerized with vinyl chloride there may be mentioned, especially, vinyl acetate and low molecular weight olefins such as, for example, ethylene and propylene.
Bulk polymerization of a monomer composition based on vinyl chloride is generally carried out in the presence of at least one polymerization initiator which generates free radicals such as, for example, an organic peroxide or an azo compound.
When the required degree of conversion of the monomer composition has been obtained, the polymerizate obtained is subjected to a degassing treatment, carried out with stirring, and intended to separate the unreacted monomer composition from the polymers or copolymers.
The pulverulent polymer or copolymer obtained after degassing the unreacted monomer composition is brought to atmospheric pressure by means of an inert gas, such as nitrogen, before its exposure to open air, which is generally followed by a screening operation.
The purpose of the screening operation, which is carried out continuously on an industrial scale, is to separate the correct product, defined as that which passes through a sieve with mesh openings of a given size, chosen as a function of the particle size distribution of the polymer or of the copolymer and of the use for which it is intended, from the coarse product consisting substantially of agglomerates of resin particles, of lower commercial value and defined as that which is retained on the said sieve.
It has already been proposed to carry out the manufacture of vinyl chloride-based polymers and copolymers by bulk polymerization or copolymerization in two stages, carried out in separate units by means of a process consisting of implementing the polymerization or copolymerization operations by ensuring that the rate of stirring of the reaction medium is as high as possible during a first stage, this being until a degree of conversion of the monomer composition of the order of 7% to 15% and preferably of the order of 8% to 12% is obtained, and then reducing the rate of stirring during the second stage of the reaction to a value which is as low as possible, but which still remains sufficient to ensure good heat exchanges within the reaction medium, this being done until the end of the said reaction. According to these methods, the two stages, the first carried out with high-turbulence stirring, and the second at a slow rate of stirring, are implemented in different autoclaves, the said stages, known as the prepolymerization stage in the case of the first one and as the final polymerization in the case of the second one, taking place in suitable units known as prepolymerizers and polymerizers. Also according to these methods, the second step is carried out using a reaction medium consisting either only of the monomer/polymer composition produced in the first stage of polymerization, or of the said monomer/polymer composition and an additional monomer composition based on vinyl chloride, identical or different from that used during the first stage, and one or more polymerization initiators.
This process and its alternative methods of implementation have been described in detail in French Patents and Certificates of Addition Nos. 1,382,072, 84,958, 84,965, 84,966, 85,672, 89,025 and Nos. 1,436,744, 87,620, 87,623, 87,625 and 87,626.
According to a particular method of implementing the process, the final polymerization operation is carried out in a vertical autoclave, described in French Patent No. 73/05,537, published under No. 2,218,350, equipped with a stirring device comprising a stirrer consisting of at least one arm matching the shape of the concave bottom of the autoclave and connected to a shaft passing through the bottom of the autoclave along its axis.
The definitions of values, concepts, and expressions used in the present application are given below, to characterize, on the one hand, the autoclave and, on the other hand, any stirrer of the type consisting of at least one arm matching the shape of the concave bottom of the autoclave and connected to a shaft passing through the bottom of the autoclave along its axis.
1. The "bottom part of the autoclave" means the volume swept by rotating the stirrer about the axis of the autoclave.
2. R denotes the radius, called "radius of the autoclave", of the cross section of the internal surface of the autoclave in the plane perpendicular to its axis passing through the radius of the bottom flange of the autoclave.
3. r denotes the radium, called "radius of the shaft", of the cross section, in a plane at the right angles to the axis of the autoclave, of the surface of the volume swept by rotating the shaft about the said axis.
4. .SIGMA..sub.1 denotes the cylindrical surface of revolution, having the autoclave axis as its axis, whose cross section in a plane (P) at right angles to the axis of the autoclave at any point O, has a radius equal to 1.05 r.
5. .SIGMA..sub.2 denotes the cylindrical surface of revolution, having the autoclave axis as its axis, passing through the point or points of the surface of the arm which are the furthest away from the said axis.
6. d denotes the radius of the cross section of the surface .SIGMA..sub.2 in the plane (P).
7. The "leading line" and "trailing line" of the arm mean the front part and the rear part, respectively, of the outline of the face of the arm, viewed from below, each part being bounded by its points of meeting with the surfaces .SIGMA..sub.1 and .SIGMA..sub.2.
8. The "front" part and "rear" part mean the front part and the rear part, respectively, in the considered direction of rotation of the stirrer.
9. (C) and (C') denotes the corresponding projections DE and D'E' of the leading line and of the trailing line of the arm in the plane (P).
10. Points D and D', known as the "central" points of the projections (C) and (C'), respectively, are situated at a distance from the point O which is equal to 1.05 r.
11. The points E and E', which are separate or which coincide, called "peripheral" points of the projections (C) and (C'), respectively, are situated at a distance d from the point O.
12. e denotes the distance from the central point D of the projection (C) to the central point D' of the projection (C').
13. The "angle of curvature" at any point F of the projection (C) means the angle O X, F Y formed by the straight line OX passing through the point F pointing in the direction from point O to point F and by the tangent FY, at point F, to the projection (C), pointing in the direction corresponding to the movement, on the projection (C), of the central point D to the peripheral point E, the said angle being taken as positive in the considered direction of rotation.
14. Using a linguistic convention, the term "tangent", at a point of the projection (C), is used regardless of whether the said point forms part of a curved section or of a rectilinear section of the said projection. In the case where the said point forms part of a rectilinear section of the said projection, the tangent to the said projection, at any point of the said rectilinear section, denotes the straight line which carried the said rectilinear section.
15. The "front face" of the arm means the front part of the arm bounded by the line of its points of meeting with the surface of the volume swept by rotating the stirrer about the axis of the autoclave and with the surface .SIGMA..sub.1.
16. The "contour" of the front face of the arm means the line of the points bounding the front face of the arm.
17. The "lower line" and "upper line" of the arm mean the lower part and the upper part, respectively, of the contour of the front face of the arm, each part being bounded by its point of meeting with surface the .SIGMA..sub.1 and by the highest point of the said contour situated on the surface .SIGMA..sub.2.
18. An "elevation" projection and an "elevation" view mean the projection, in any plane (Q) passing through the axis of the autoclave, obtained by folding back, in the said plane (Q), alone the circular arcs centered on the axis of the autoclave.
19. (G) and (G') denote the respective elevation projections HJ and H'J of the lower line and of the upper line of the arm in the plane (Q), the said elevation projections being called, respectively, more briefly, "projection" (G) and "projection" (G').
20. The points H and H', called the "central" points of the projections (G) and (G'), respectively, are situated at a distance from the axis of the autoclave which is equal to 1.05 r.
21. the point J common to the projections (G) and (G'), called the "extreme peripheral" point of the projection (G) and/or of the projection (G'), is situated at a distance d from the axis of the autoclave.
22. h denotes the distance from the joint J to the radius of the autoclave flange, which distance is measured in the vertical direction and taken as positive from the bottom upwards starting from the radius of the flange, taken as origin.
23. e' denotes the distance from the central point H of the projection (G) to the central point H' of the projection (G').
24. S denotes the surface area of the surface bounded by the elevation projection of the contour of the front face of the arm.
25. (s) denotes the cross section of the surface of the arm in a plane at right angles to its leading line at any point M.
26. M', N and N' denote the points of meeting of the cross section (s) and, respectively, of the trailing line and of the lower and upper lines of the contour of the front face of the arm.
27. a.sub.1, a.sub.2, a.sub.3, and a.sub.4 denote the apexes of the rectangle, with two horizontal sides, circumscribed on the cross section (s).
28. a.sub.1, a.sub.2, a.sub.3 a.sub.4, a.sub.1 a.sub.4 and a.sub.2 a.sub.3 denote the sides of the said rectangle on which the points M, M', N and N' are situated, respectively, whatever the shape of the cross section (s).
29. Each of the arms of any stirrer of this type employed until now, called, in the present description, "stirrer Bo", has the following characteristic: the projections (C) and (C') of its leading line and of its trailing line, respectively, are rectilinear, parallel and arranged so that the point O is situated between the straight lines which produce it. Whatever the considered direction of rotation of the said stirrer Bo, the angle of curvature .alpha. at any point F on the projection (C) is negative. When rotating in either of the two directions of its rotation, the prior art said stirrer Bo drives the medium with a centrifugal motion; away from the axis, in the lower part of the autoclave.
The power consumed by the prior art stirrer Bo at certain critical stages of the process, especially at the end of the polymerization operation and during the degassing treatment of the polymerizate, makes it necessary to provide the said stirrer with a relatively large drive unit.